During semiconductor processing of substrates, it is necessary to transport substrates into and out of semiconductor processing chambers, which is typically done with a wafer handling robot of some type. As used herein, the terms “wafer” and “substrate” are used interchangeably and may be used to refer to semiconductor or glass substrates. A typical wafer handling robot may have a multi-joint arm that is configured to independently extend, retract, rotate and, in many cases, raise and lower so as to transport substrates between one or more semiconductor processing chambers, or a loadlock of a transfer chamber leading to one or more semiconductor processing chambers, and one or more load ports or load stations. Such a wafer handling robot may include a thin, blade- or spatula-like end effector that may be positioned beneath a substrate and that has a plurality of contact pads or other points configured to contact the underside or edge of the substrate when the end effector is raised up into contact with the substrate. The end effector is typically designed to only contact the substrate at these locations to reduce the amount of contact between the end effector and the substrate, thereby lessening the opportunities for particulate generation and damage to the substrate.
A typical semiconductor processing tool may be designed to process large numbers of substrates, with multiple substrates being moved through the tool simultaneously. For example, many semiconductor processing tools include a plurality of semiconductor processing chambers arrayed around a central hub, which may be referred to as a transfer chamber. Each semiconductor processing chamber may be connected to the transfer chamber by way of a gate valve or slit door interface that allows that semiconductor processing chamber to be sealed off from the transfer chamber. A vacuum-side wafer handling robot may be located within the transfer chamber and configured to move substrates between the various attached semiconductor processing chambers.
The transfer chamber may also be connected with one or more load locks, which may serve as airlocks that separate the transfer chamber (and the semiconductor processing chambers) from the ambient environment of the processing facility in which the semiconductor processing tool is located. The load locks allow the transfer chamber (and the semiconductor processing chambers) to be operated at a very lower pressure, e.g., in the milliTorr range, continuously while still allowing substrates to enter and exit the transfer chamber from the ambient environment.
The load locks may be connected with an equipment front end module (EFEM), which is a large, typically enclosed structure that may include an atmospheric-side wafer handling robot. The EFEM may also include one or more load ports, which are interfaces through which substrates may enter and exit the EFEM as part of the flow of substrates through the semiconductor processing facility housing the EFEM. The atmospheric-side wafer handling robot may be configured to transfer substrates between the load port(s) and the load lock(s).
In a typical semiconductor processing facility, the movement of substrates between semiconductor processing tools is accomplished through the use of front-opening unified pods (FOUPs), which are sealable containers with vertically-arranged shelves for supporting a large number, e.g., 25 or 30, substrates at a time. A FOUP may be docked at an EFEM load station, and the substrates contained therein may be removed from the FOUP by the atmospheric-side wafer handling robot and transferred to the load lock for retrieval by the vacuum-side wafer handling robot and transfer to/between the semiconductor processing chambers. At the conclusion of the semiconductor processing operations involving a particular substrate, the substrate may be removed from the transfer chamber and returned to the FOUP or to another FOUP in a similar manner.
As is evident, there may be multiple periods of time while a substrate is resident within a particular semiconductor processing tool in which the substrate may be moved about the semiconductor processing tool by a wafer handling robot. Typically, it is desirable to perform such wafer transport operations as quickly as possible since time spent transporting substrates is time during which the substrate is not subject to actual semiconductor processing operations. In semiconductor processing operations, through-put is of paramount importance, and semiconductor processing tools are typically configured to minimize (or reduce) the total amount of time that any given substrate spends within the semiconductor processing tool.
Thus, while wafer handling robots typically move quite quickly, they are typically controlled so as to avoid exposing the substrates that they transport to more than a predetermined level of acceleration to avoid causing the substrates to overcome the friction forces that hold them in position on the end effector contact pads. If the accelerations experienced by a substrate are too great and overcome the contact pad friction forces, the substrate may slip and become misaligned or may, in a worst case, fall off the end effector, resulting in loss of the substrate.
Commercial products exist that allow measurement of the accelerations experienced by a substrate during wafer handling operations. Such products typically take the form of a test wafer having a single tri-axial accelerometer and configured to report out wafer center accelerations in the X- and Y-directions.